It is known from the EP-A1-0 688 951 to provide an injection control for small, compact batteryless four-stroke engines, in which an electromagnetic injection valve is operated by means of voltage pulses, which are induced by a co-rotating permanent magnet mounted on a flywheel in a coil assembly rigidly mounted in the proximity of the flywheel. While the injection start is unchangeably determined by the position of the coil assembly, the injection duration is limited by a timing control circuit according to a computed period of time by an interruption of the injection valve circuit. The problems which arise when such an injection valve circuit is interrupted are described in the EP-B1-0 543 826. Whereas four-stroke engines generally possess a relatively constant engine running over the entire speed and load range, two-stroke enginesmanifest significant differences between the speed ranges "idling" and "operative range". In the idle running (low speeds of some 100 r.p.m.), a considerably rough running exists which always calls for an accurate injection. If, starting out from a certain trigger signal, a computation of the injection start is performed, the same has to take place within the immediate proximity of the trigger signal since otherwise serious errors with regard to the desired injection start may occur. As already mentioned, these depend essentially upon the degree of regularity of the running of the engine and may differ considerably from revolution to revolution so that the running of the engine becomes uncontrollable. That is why also the injection has to take place in the immediate proximity of the trigger point or trigger signal.
As is depicted in the FIG. 1 in an angular diagram of the engine crankshaft (KW) in relation to the upper compression point (OT), for lower speeds the injection starts (ESB), in dependence upon the respective engine configuration, are located in a first (shaded) range (ESB1) at between 180 and 240.degree. KW (angle W4 and W3) before the upper compression point (OT) of the internal combustion engine. At higher speeds (several 1000 r.p.m.), other injection starts, depending on the respective load states, are necessary. Usualyl, a displacement of the injection start in the direction "Early" into a second (shaded) range (ESB2) is necessary, which is located between 270 and 350.degree. KG (angle W2 and W1) before the upper compressions point (OT).
For low speeds (e.g. when idling), according to the foregoing explanations it is both expedient and advantageous to place the trigger point at which the trigger signal is generated, close to the beginning of the first range (ESB1), thus e.g. at the point identified with (TP) in FIG. 1. In this way merely a short angular distance exists between (TP) and the start of the injection area (W3). When the injection start, at increasing speeds, is now displaced forwardly from the range (ESB1) into the range (ESB2), the rtigger point (TP) should actually also be displaced accordingly. A (quasi) displacement of the trigger point does expediently take place in that the computation of the injection start is performed for the next revolution (long arrow from TP via the angles W5 and W6 of the ignition area ZB to W1 in FIG. 1). On account of the requisite computation time of the period necessary for controlling the opening time of the injection valve, it may, in dependence upon the required injection start, happen that the trigger signal being used for the idling range (e.g. of a Hall transmitter) cannot be evaluated. Because of this, the injection for the next revolution would not be possible, or only strongly subject to errors.